This invention relates to the disposal of municipal solid waste and, more particularly, to a pollution-free process and apparatus for disposing of municipal solid waste and generating electricity.
The disposal of municipal solid waste (MSW) presents an increasingly significant and difficult problem for municipalities to solve. As space for landfills diminishes and the cost of such waste disposal has become prohibitive, many schemes have been devised to burn the waste and generate electricity.
The composition of MSW can vary from locale to locale, but, on the average, has been found typically to be about 50% of combustible materials, such as wood, paper, plastic, food, yard waste and textiles, 5% metals, 10% glass, dirt and stones, and 35% water. The water content varies between 40% and 60% of the waste. To burn efficiently, the non-combustible content should be removed, along with the majority of the water. Sorting and magnetic separation can remove a majority of the unwanted components, but it is necessary to reduce the water content to 10% or below.
Many schemes have been devised to dewater the MSW, including heating, centrifugal separation, compression. However, these schemes have proved to be extremely costly since they require a significant amount of fuel just to heat the waste to dewater it sufficiently to become burnable. In addition, the heating of the wet waste produces a significant amount of airborne contaminants or pollutants. Scrubbers and other costly equipment are necessary to reduce the level of pollution to tolerable amounts. As a result of these shortcomings, hundreds of so-called xe2x80x9ctrash-burning power plantsxe2x80x9d have become economically untenable and hundreds more are run at a significant loss.
More recently a process has been introduced which utilizes composting and aerobic techniques to dry the MSW, using closely controlled humidity and temperature. However, this process is so slow that several days are required to reduce the water content sufficiently to produce a burnable biomass.
There is a need for a process and apparatus for disposing of MSW that is cost-efficient and non-polluting.
It is therefore an object of this invention to a process and apparatus for disposing of MSW that is cost-efficient and non-polluting.
In one aspect, this invention features apparatus and a process for reducing the moisture content of MSW by continuously passing warm dry air through the waste in a closed chamber without appreciably heating the waste.
In another aspect, this invention features a method of processing municipal solid waste, comprising the steps of: removing identifiable unwanted components from the waste, chopping the waste into small pieces of a size suitable for handling and combustion, reducing the moisture content of the waste to a predetermined amount in a closed system by passing dry air through the waste in a confined space to absorb moisture and produce moist air, dehumidifying the moist air via a refrigeration cycle and recycling the dehumidified air through the waste repeatedly in a closed system until the water content has reached the predetermined amount, combusting the waste to produce heat, producing steam from the heat, and using the steam to produce electricity, thereby eliminating any transfer of contaminants from the waste to the atmosphere during dewatering of the waste.
Preferably, MSW moisture content is reduced to 10% or less, and the steam is used initially to generate electricity, and thereafter is used to preheat the dehumidifying air to enhance its ability to absorb moisture.
In a further aspect, this invention features a system for disposing of municipal solid waste, comprising means for removing unwanted content from the waste, means for chopping the waste into pieces small enough for handling and combustion, a closed drying chamber for reducing the moisture content of the waste, including means for injecting dry air through the chamber to absorb moisture from the waste and producing dewatered waste, means for removing moist air from the drying chamber, refrigeration cycle means for dehumidifying the air and recycling the dehumidified air back through the chamber in a dosed system, a combustion chamber for combusting the dewatered waste to produce heat, a heat exchanger for using the heat to produce steam, and a steam-powered electric generator for utilizing the steam to produce electricity, whereby no contaminants are transferred to the atmosphere during the dewatering of the waste.
Another feature of this invention is provision of a second heat exchanger for transferring heat from the steam exiting the generator to the dry air before it enters the drying chamber.